High voltage electrical equipment, for example industrial electrical equipment, often uses a three phase plus neutral electrical power distribution arrangement. In such an arrangement, the neutral is referred to as an external neutral, because it originates at the equipment being supplied (the load) and is fed back to a distribution panel or the like. Typically, in order to provide ground fault monitoring and protection, a current transformer is used to sense the amount of current flowing in the external neutral. During normal operation, this current should be zero. During fault conditions, this current may be hundreds of amperes.
Known external neutral current transformers (ENCTs) are connected in series with the external neutral and produce a voltage signal indicative of the amount of current flowing through the external neutral. This voltage signal is connected to the trip unit of a circuit breaker supplying electrical power to the load. During an external neutral fault condition, abnormal current in the external neutral is sensed by the ENCT, which applies a voltage signal to the trip unit of the circuit breaker to cause the circuit breaker to open the circuit and discontinue the supply of electrical power to the load.
If an ENCT is highly accurate, it may be used for other applications beside fault protection, including power metering, for example.